Heretofore, workpieces have been positioned within an aerated or fluidized plastic bed and vibrated for a predetermined period of time in order to provide a plastic coating on the workpieces. The workpieces are usually heated to a predetermined high temperature before being lowered into the aerated plastic coating which may be formed of a corrosion-resistant powdered material, and the powdered material is melted by the heated workpiece. However, it has been difficult to provide a uniform plastic coating of a predetermined thickness, particularly with certain types of workpieces, such as coiled springs.